The present invention relates generally to an image sensor, and, more particularly, to a CMOS image sensor and/or a charge coupled device (CCD), which is capable of decreasing leakage current between photodiodes and a method for fabricating the same.
As is well known, an image sensor is an apparatus for sensing light reflected from an object and for generating image data. In particular, an image sensor fabricated by using CMOS (Complementary Metal Oxide Semiconductor) technology is called a CMOS image sensor.
Generally, the CMOS image sensor includes a plurality of pixel units having a light sensing region and a peripheral circuit region. Each of the pixel units also includes a light sensing element formed on the light sensing region and a plurality of transistors formed on the peripheral circuit region. The light sensing elements, such as a pinned photodiode, senses incident light reflected from an object and accumulates photoelectric charges that are generated due to the incident light. The transistors control the transfer of the photoelectric charges.
A prior art method for fabricating a photodiode region of a conventional image sensor will be described with reference to FIGS. 1A and 1B.
As shown in FIG. 1A, field oxide layers 102 for isolating devices are formed in a p-type silicon substrate 101. The field oxide layers 102 are formed by a LOCOS (Local Oxidation of Silicon) or an STI (Shallow Trench Isolation) method. Thereafter, P-type field stop layers 103 are formed beneath the field oxide layers 102 to prevent the generation of leakage current between devices.
As shown in FIG. 1B, after forming an ion implantation mask (not shown) to open the photodiode regions 3a and 3b on the p-type silicon substrate 101, a low-concentration and high-energy ion implantation is performed to form n-type doping regions 104 in the p-type silicon substrate 101. Thereafter, a high-concentration and low-energy ion implantation is performed to form p-type doping regions 105 beneath the surfaces of the p-type silicon substrate 101. After that, a thermal treatment process is performed to diffuse the impurities in the n-type doping regions 104 and the p-type doping regions 105, whereby pinned photodiodes are formed. Each pinned photodiode includes a p-type doping region 105, an n-type doping region 104 under the p-type doping region 105 and the p-type substrate 101 under the n-type doping region 104.
The pinned photodiode has several merits in comparison with a source/drain PN junction type diode and a MOS capacitor type diode. For example, the n-type doping region of the pinned photodiode having a PNP structure can be fully depleted and the depth of the depletion layer can be increased. Thereby, an incident photon can easily generate an electron. That is, the quantum efficiency and the light sensitivity of the pinned photodiode is improved relative to the aforementioned diodes.
A major shortcoming of the conventional CMOS image sensor is the generation of leakage current (LKG) between pinned photodiodes formed in adjacent pixel units. Although, the field stop layers 103 are formed beneath the field oxide layers 102, the generation of the leakage current (LKG) cannot be prevented effectively because of the doping regions 104 formed by the high-energy ion implantation.
If the doping concentration of the field stop layers 103 is increased to prevent the generation of the leakage current, then the impurities in the field stop layers 103 are diffused to the photodiodes. As a result, the characteristics of the photodiodes are deteriorated.
In accordance with an aspect of the invention, an image sensor is provided which contains a first pixel unit and a second pixel unit, each of the first and second pixel units having a photodiode region. The image sensor includes a semiconductor substrate of a first conductivity type; a device isolation layer formed in the semiconductor substrate; a field stop layer formed beneath the device isolation layer; a trench formed in the semiconductor substrate between the photodiode region of the first pixel unit and the photodiode region of the second pixel unit; a first doping region of the first conductivity type formed beneath the surface of the semiconductor substrate of the first photodiode region of the first pixel unit; an insulating member within the trench; and a second doping region of a second conductivity type formed in the semiconductor substrate under the first doping region.
In accordance with another aspect of the invention, an image sensor is provided which contains a first pixel unit and a second pixel unit, each of the first and second pixel units having a photodiode region. The image sensor includes: a semiconductor substrate of a first conductivity type; a device isolation layer formed in the semiconductor substrate; a field stop layer formed beneath the device isolation layer; a trench formed in the semiconductor substrate between the photodiode region of the first pixel unit and the photodiode region of the second pixel unit; an insulating layer covering a surface of the field oxide layer and a surface of the semiconductor substrate including a surface of the trench; a conducting member within the trench, wherein the conducting member covers the device isolation layer and exposes the semiconductor substrate of the photodiode region of the first pixel unit; a first doping region of the first conductivity type formed beneath the surface of the semiconductor substrate of the photodiode region of the first pixel unit; and a second doping region of a second conductivity type formed in the semiconductor substrate under the first doping region.
In accordance with another aspect of the invention, a method for fabricating an image sensor having a first pixel unit and a second pixel unit is provided, each of the first and second pixel units having a photodiode region, the method comprising the steps of: providing a semiconductor substrate of a first conductivity type; forming a device isolation layer in the semiconductor substrate; forming a field stop layer beneath the device isolation layer; forming a trench in the semiconductor substrate between the photodiode region of the first pixel unit and the photodiode region of the second pixel unit; forming a first doping region of the first conductivity type beneath the surface of the semiconductor substrate of the photodiode region of the first pixel unit; forming an insulating member within the trench; forming a second doping region of a second conductivity type in the semiconductor substrate under the first doping region; and performing a thermal treatment process to diffuse impurities in the first doping region and the second doping region.
In accordance with still another aspect of the invention, a method for fabricating an image sensor having a first pixel unit and a second pixel unit is provided, wherein each of the first and second pixel units has a photodiode region, the method comprising the steps of: providing a semiconductor substrate of a first conductivity type; forming a device isolation layer in the semiconductor substrate; forming a field stop layer beneath the device isolation layer; forming a trench in the semiconductor substrate between the photodiode region of the first pixel unit and the photodiode region of the second pixel unit; forming an insulating layer on a surface of the device isolation layer and on a surface of the semiconductor substrate including on a surface of the trench; forming a conducting member within the trench and covering the insulating layer formed on the device isolation layer; forming a first doping region of the first conductivity type beneath the surface of the semiconductor substrate in the photodiode region of the first pixel unit; forming a second doping region of a second conductivity type in the semiconductor substrate under the first doping region; and performing a thermal treatment process to diffuse impurities in the first doping region and the second doping region.